This invention relates generally to a digital logic circuit and, in particular, to a magnitude comparison circuit for indicating alternatively the relative magnitudes of two pairs of binary numbers or a single pair of binary numbers.
In the digital computer art and in other arts where digital numbers are processed, compared, and otherwise manipulated, it is often necessary to compare one binary number with another and to indicate whether one of the numbers is greater than, equal to, or less than the other number. For example, it is common in programs for controlling the operation of digital computers to provide loops or conditional branches whose execution depends upon the outcome of a comparison of the relative magnitudes of two numbers. Therefore, appropriate circuitry must be provided in the processor of the data processing system for comparing two binary numbers and indicating whether one is greater than, equal to, or less than the other.
Digital comparison circuits are known in the prior art for comparing two binary numbers each comprising one or more bits. Integrated Electronics: Analog & Digital Circuits & Systems, Millman & Halkias, McGraw-Hill Book Company, New York, 1972, p. 605 et seq. describes such a digital comparator. The prior art magnitude comparison circuit does not make effective use of its full binary number comparison capacity since it is designed for comparing two and only two binary numbers at any given time. In order to provide in a processor utilizing the prior art magnitude comparison circuit the capability of simultaneously comparing two pairs of binary numbers it is necessary to provide redundant magnitude comparison circuits. The capability of simultaneous comparison of of two pairs of binary numbers is desirable from the standpoint of increasing the processing speed; however, the provision of redundant magnitude comparison circuits which may not always be fully utilized increases the cost and size of the processing hardware.